The present invention relates to the field of printers and, more particularly, to a low cost ink drop detector for use in a drop-on-demand printer.
Prior printers including black and white and color printers commonly include at least one printhead that ejects ink drops onto paper. Such printheads may include multiple nozzles through which ink drops are ejected. A printhead may eject ink in response to a drive signal generated by print control circuitry in the printer. A printhead that ejects ink drops in response to drive signals may be referred to as a drop-on-demand printhead.
An inkjet printhead is an example of a drop-on-demand printhead. Inkjet printheads are capable of forming an image on different types of media. The inkjet printhead may eject droplets of colored ink through a plurality of orifices or nozzles onto a given media, such as paper, as the media is advanced through a xe2x80x9cprintzonexe2x80x9d or platen area. The printzone may be defined by the planar area that is accessible by the printhead orifices due to any scanning and/or reciprocating movement of the printhead in relation to the media. Examples of methods for expelling ink from the printhead orifices, or nozzles, include known piezo-electric and thermal techniques. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company, and hereby incorporated in their entireties by reference.
In order to achieve a high level of image quality in an inkjet printing mechanism, it is often desirable that the printheads exhibit several qualities, including: consistent and small ink drop size, consistent ink drop trajectory from the printhead nozzle to the print media, and inkjet nozzles that do not easily clog. To this end, inkjet printing mechanisms may include a service station for the maintenance of the inkjet printheads. These service stations may include scrapers, ink-solvent applicators, primers, and caps to help keep the nozzles from drying out during periods of inactivity. Additionally, inkjet printing mechanisms may contain service routines that are designed to exercise the printhead by firing ink out of each of the nozzles into a waste spittoon in order to prevent the formation of dried ink resulting in nozzle clogging.
Despite these preventative measures, there are many factors at work within an inkjet printing mechanism that may clog the inkjet nozzles, resulting in inkjet nozzle failures. For example, paper dust particles may collect on and eventually clog the nozzles. Ink residue from ink aerosol or from partially clogged nozzles may be spread by service station printhead scrapers into open nozzles thereby clogging additional nozzles. Accumulated precipitates from the ink inside of the printhead may also occlude the ink channels and the nozzles. Additionally, heater elements in a thermal inkjet printhead may fail to energize thereby causing the nozzle to fail.
Clogged or failed printhead nozzles may result in objectionable and easily noticeable print quality defects such as banding (visible bands of different hues or colors in what would otherwise be a uniformly colored area) or complete color voids in the image. In fact, inkjet printing systems are so sensitive to clogged nozzles, that a single clogged nozzle out of hundreds may be noticeable and objectionable in the printed output.
Prior printers typically lack a mechanism for determining whether the print head actually requires cleaning. Such printers may apply a service station to the print head based on a determination that the print head may possibly require cleaning. Unfortunately, such printers must then employ periodic cleaning, rather than cleaning when necessary, that usually slows the overall printing throughout and may result in unnecessary maintenance ink loss and wear, or may fail to prevent a failure if performed too infrequently.
In order to detect whether an inkjet printhead nozzle is firing, a printing mechanism may be equipped with a low cost ink drop detection system, such as the one described in U.S. Pat. No. 6,086,190, which is assigned to the present assignee, Hewlett-Packard Company, and is incorporated herein by reference in its entirety. This drop detection system utilizes an electrostatic sensing element that is imparted with an electrical stimulus when struck by a series of ink drop bursts ejected from an inkjet printhead.
In practical implementation, however, this electrostatic sensing element may have some limitations. The sensing element may adversely react with ink residue formed as a result of contact with the ink drop bursts. Additionally, drop detect signals provided from the sensing element to the sensing electronics may easily be subjected to noise due to their relatively small amplitudes. Furthermore, the ink residue remains conductive and may short-circuit the sensing electronics.
Another possible method for detecting the ejection of ink drops from a print head is to equip the printer with a drop detection station that employs piezo-electric material and associated circuitry that detects the impact of the ink drops hitting the detection station. Unfortunately, such piezo-electric material is relatively expensive and adds to the manufacturing cost of a printer. In addition, such a mechanism usually cannot detect extremely small ink drops that are used in high resolution and color printers. Moreover, piezo-electric material may lose sensitivity as ink accumulates on its surface thereby reducing its ability to detect ink drop impacts.
Another possible solution is to equip the printer with an optical detector that includes a light source and a detector. An ink jet nozzle may be aimed so that ink drops pass between the light source and the detector and occlude light rays that travel between the light source and the detector. The circuitry for such an optical detector may unduly add to the manufacturing cost of a printer. In addition, such a technique may require very fine control over the positioning of the optical detector with respect to nozzles being tested. Moreover, mist or spray from the nozzle may contaminate the optical detector and cause reliability problems.
Another possible solution that is specific to thermal ink jet print heads is to equip the print head itself with an acoustic detector. Such an acoustic drop detector may detect the shock wave associated with the collapse of ink bubbles in the print head. Such ink bubble shock waves may, however, occur even though ink is not being ejected from the print head. In addition, acoustic measurements may be corrupted by large current pulses that occur during printer operation. Moreover, the acoustic detector and associated signal amplifier circuitry for such an acoustic detector may unduly increase the overall manufacturing costs of a printer.
Therefore, it would be desirable to have a sensing element that have substantial immunity from the potentially harmful effects of ink residue and that may be easily integrated into various printing mechanism designs. It would also be desirable to have a method of efficiently and economically constructing such a sensing element. It would also be desirable to have a more effective system for cleaning inkjet nozzles.
The present invention is directed to a system and method for a waste ink removal apparatus for cleaning ink residue from an ink drop detection sensor in a printing mechanism, including an assembly pivotally supported by a pivot, the assembly pivoting between a first orientation and a second orientation; an ink drop sensor located on the assembly; a pivoting device connected to the assembly and an absorbent pad positioned to contact the ink drop sensor when the assembly is in the second orientation, wherein operation of the pivoting device causes the assembly to pivot between the first and second orientations such that waste ink is removed from the ink drop sensor when the assembly is in the second orientation.